1. Field of the Invention:
The present invention pertains generally to the field of radiation detecting devices and, more particularly, to the field of real-time digital radiation imaging devices.
2. Discussion of the Background:
There are several instances in modern megavoltage radiotherapy and diagnostic x-ray imaging where real-time imaging of high energy photons is a highly useful and critically important technique.
In external beam megavoltage radiation therapy, high energy beams of gamma rays or X rays are used to irradiate a target volume containing tumorous tissue. These high energy photons are typically obtained from either a radioactive 60-Co source (1.17 MeV and 1.33 MeV gamma rays) or produced by means of an accelerator which generates x-ray bremsstrahlung photons beams with energies from 3 MV to 50 MV. In such therapy, it is highly desirable that the maximum dose be delivered to the target volume and the minimum dose be delivered to the surrounding tissue. Prior to treatment, which typically consists of irradiating the patient on a daily basis for several weeks, the patient undergoes a number of preparatory steps in order to identify the region to be irradiated and to determine a "treatment plan" specifying exactly how this irradiation is to be performed. Often, one of these steps is to place the patient on a "treatment simulator", which simulates the motions and geometry of the therapy machine, and which provides fluoroscopic and radiographic diagnostic x-ray images of the patient. The simulator thus provides a means to simulate a treatment to a patient using diagnostic x-rays in place of megavoltage radiation. During simulation the fluoroscopic x-ray images provide a real-time means of simultaneously observing patient anatomy while the patient position is manipulated. In this fashion, a desired patient orientation with respect to the simulated treatment beam is achieved. After simulation, x-ray images recorded during the simulation can then be used to develop a treatment plan for the patient. The goal of this treatment plan is to decide exactly how to perform the actual treatment, i.e., what geometric and dosimetric combination of megavoltage beams to use to satisfactorily irradiate the target region but spar the surrounding normal tissues. Once a treatment plan has been determined, often with the assistance of a computer which allows, among other things, manipulation of the simulation information as well as CT or other imaging information, the patient is typically taken back to the simulator for a verification-simulation in order to verify the geometric correctness of the plan.
When the patient is brought into the treatment room, it is highly desirable, prior to treatment, to verify that the orientation of the patient with respect to the treatment beam closely coincides with the setup achieved in the simulator room. Once verified, the prescription dose can be delivered to the target volume. The achievement of this goal is complicated by the fact that the patient anatomy moves due to both voluntary and involuntary patient motions. Such complications encourage the possibility of delivering too little dose to the target region and/or overdosing the surrounding tissues. In addition, for treatment machines which use a computer controlled scanning treatment beam, there is the additional uncertainty of whether the beam is correctly directed on a burst by burst basis.
The above problems can be overcome by real-time imaging of the megavoltage photon beam. Several real-time imagers are being developed around the world. A real-time megavoltage imager has been developed by H. Meertens at the Netherlands Cancer Institute in Amsterdam which is disclosed in European Patent Application 0196138 which corresponds to U.S. Pat. No. 5,019,711. The Meertens' device operates on the principle of a scanning liquid ionization chamber. However, the Meertens' device is able to detect the imaging signal only over a fraction of the field of view at a given time.
Radiation detecting devices are taught in U.S. Pat. Nos. Hynecek, 4,679,212; Luderer et al., 4,250,385; DiBianca, 4,707,608; Haque, 4,288,264; Kruger, Re. 32,164; Barnes, 4,626,688; and DiBianca et al., 4,525,628; however, these detectors do not make possible real-time imaging for megavoltage photons.
Efforts to develop imagers based on camera-fluoroscopy combinations have produced images of greatly varying quality at rates ranging from four images a second to one image every eight seconds. Such systems have the merit that they are able to detect the imaging signal over the entire field of view simultaneously. However, a camera's expensive and delicate imaging electronics would be irreversibly damaged after approximately 10 to 130 kilorads of dose. Thus, a mirror is used to reflect the light produced by a metal-phosphor screen combination to a camera sitting outside of the direct radiation field. This makes necessary the presence of a bulky light box located in the vicinity of the treatment table where such obstructions are highly undesirable. Furthermore, as the camera is located 2 to 3 feet from the screen and as the target of the camera is small relative to the screen, only a very small amount of light emitted by the screen is utilized by the camera, less than 1%. Consequently, the image quality is limited by the light collection stage rather than by the high-energy quanta detected in the metal-phosphor screen which results in images that are less than optimal.
Recently, another camera-fluoroscopy megavoltage imager consisting of tightly packed, tapered, optical fibers has been reported in Int. J. Radiation Oncology Biol., Phys., Vol 18, pages 1477 to 1484. The fibers make up a 40.times.40 cm.sup.2 surface, 12 cm thick, which sits in the beam behind a metal-phosphor screen and "pipes" the light to a video camera. The optical fibers are bunched together in bundles of 1.5.times.1.5 cm.sup.2 at the input end and the imager has a thickness of 12 cm. Unfortunately, this system has a light collection efficiency no greater than that of the mirror-camera systems and, like those systems, is rather bulky.
In the optical megavoltage imaging systems discussed above, considerably less than 1% of the visible light photons emitted by the phosphor layer are converted into signal. As a direct consequence, the quantum sink is the light collection stage rather than the stage where X rays are converted to high-energy electrons which enter into the phosphor. Thus, the quality and speed of imaging in the above systems are adversely affected. Moreover, the bulkiness of both the camera-mirror and the optical fiber fluoroscopic imaging systems compromises the clinical utility of these imaging devices.
In diagnostic x-ray imaging, the object to be imaged is placed between the x-ray source and an x-ray receptor. The X rays are generated by an x-ray tube and the range of x-ray energies used corresponds to peak tube voltages of .about.20 kVp to .about.150 Kvp.
Diagnostic x-ray imaging may be divided into two modes, radiographic and fluoroscopic. In current radiographic imaging, single or multiple x-ray images are recorded from an x-ray irradiation in a fashion requiring some intermediate step (such as film development) before the image or images can be viewed. Thus the image or images are not available for presentation immediately after the irradiation, i.e., they are not available within a few seconds. Since the object to be imaged may be subject to motion, the objective of such imaging is usually to capture each image in an interval sufficiently short so as to freeze the motion. In fluoroscopic imaging, a series of consecutive images are produced and presented to an observer during the course of an irradiation allowing the object to be imaged in real-time.
In a similar fashion, radiotherapy imaging may also be divided into radiographic and fluoroscopic modes. A single image of the patient may be obtained prior to the main daily treatment with an amount of radiation small compared to the main treatment dose. Alternatively, a single image may be obtained using the entire treatment dose. Both of these forms of imaging are essentially radiographic modes of acquiring an image. Alternatively, a series of consecutive images may be obtained and displayed during irradiation with all or a fraction of the treatment dose. This is clearly a fluoroscopic mode of imaging.
Radiographic and fluoroscopic x-ray imaging (also called radiography and fluoroscopy) have been under continual development since the discovery of X rays in 1895. Many summaries of the developments in this field exist and a concisely written recent one is to be found in RadioGraphics, Volume 9, Number 6, November 1989.
Currently, most diagnostic radiographic imaging is performed using film-screen systems in which x-ray film is placed next to one or between two phosphor screens which convert the X rays to light which exposes the film. The film then must be developed and the image is then viewed directly from the film and/or the film may be digitized for presentation on a monitor. A second manner of producing radiographic images is by means of so-called scanning-laser-stimulated luminescence (RadioGraphics, Volume 9, Number 6, November 1989, page 1148). In this method, plates containing photostimulable phosphors are irradiated in a manner analogous to film. The phosphors are then "read out" by means of a laser with direct conversion of the signal to digital form. Both film and photostimulable phosphors offer practical and useful means of acquiring radiographic images. However, neither method allows presentation of the image immediately after the irradiation since time is required either for film development or for laser scanning. Film development typically takes .about.90 seconds while laser scanning takes several minutes and several more minutes can be spent going to and from the film processor or laser scanner. Moreover, while an x-ray tube may be quite portable, the film processor and laser scanner are far less so. Finally, for high quality film development, the temperature and quality of the chemicals in the film processor must be closely monitored. It would be highly desirable to develop an imaging technology which allowed immediate presentation of the radiographic image after irradiation meaning within a couple of seconds. This would provide quick feed-back to the operator taking the image letting the operator immediately judge whether the image is of sufficient quality or whether a retake is necessary. This would greatly reduce the time and expense associated with radiographic imaging. Furthermore, an electronic imager could have built-in controls so as to help assure that the irradiation produced a near-optimal image every time thereby reducing the frequency of retakes and thus reducing the total dose to the patient. A variety of attempts to develop electronic means of acquiring digital diagnostic images have been reported with perhaps the most promising involving a scanning linear array of photodiodes and collimation (RadioGraphics, Volume 9, Number 6, November 1989, page 1148). Thus far, such devices have not been adapted for routine clinical use.
Diagnostic fluoroscopic imaging is currently practiced using an x-ray image intensifier tube (RadioGraphics, Volume 9, Number 6, November 1989, pages 1137, 1138). The image intensifier tube converts incident radiation to light, typically using a CsI screen. Then, through a series of steps, an amplification of the light is achieved and the light output from the image-intensifier tube is captured and converted to an electronic image by means of a camera, a CCD, or some similar device. An x-ray image-intensifier tube along with a camera, or a CCD, or some similar device constitutes an x-ray image-intensifier (XRII) fluoroscopic imaging unit. While XRII units are very useful fluoroscopic imaging devices, they suffer from a number of serious deficiencies. First, XRII units are relatively bulky having a length usually exceeding 50 cm. This is a definite hindrance in various clinical procedures. For example, XRII units restrict the possible motions of radiotherapy simulators thereby limiting the treatment positions that can be simulated. Restrictions also occur in a variety of diagnostic x-ray procedures due to the bulk of the XRII unit. Second, the quality of the images are compromised by various well-known effects associated with image intensifiers and cameras which lead to distortions and glare. Furthermore, XRII units are easily affected by stray magnetic fields and are generally difficult to maintain at peak imaging performance. It would be very useful to have an alternative technology, which is thin, light-weight, and free of distortion, glare, and the effects of magnetic fields suffered by XRII units. Furthermore, such an alternative technology would allow for a portable fluoroscopic imager offering high quality images. This possibility is generally not practical with XRII units.
From the above discussion, it is clear that fluoroscopic imaging with both megavoltage X rays and gamma rays and diagnostic quality X rays involves production and presentation of consecutive images during the course of an irradiation. Consequently, such imaging is inherently real-time in nature. Further, radiographic imaging as practiced with technologies such as film-screen systems and photostimulable phosphors requires some time consuming intermediate step (such as film development or laser-scanning) before the image is available for presentation. Such imaging is thus not real-time in nature. A variation of diagnostic radiographic imaging occurs when a series of x-ray films are rapidly exposed, one after the other, or on a continuous roll of film. However, since the images are not available for presentation during, or immediately after, the irradiation, this is also not real-time imaging. However, if a practical technology could be developed which would provide presentation of high quality radiographic images immediately after irradiation, this would then be another form of real-time imaging. Moreover, if this alternative imager offered the images in digital form, this would be of considerable utility as it would facilitate electronic processing and presentation of the images as well as electronic archival and transfer. As outlined above, the existence of a technology offering real-time radiotherapy and/or diagnostic digital imaging would be of definite benefit.
In selecting the materials for a real-time imager for megavoltage photon radiation therapy and diagnostic x-ray imaging, care must be taken that the materials can withstand high levels of radiation exposure over long durations of time. Another consideration is that the radiation detecting elements be arranged over a relatively large surface area. For instance, a detection surface of at least 25.times.25 cm.sup.2 is necessary for head and neck portals in radiation therapy. For pelvic, abdominal and thoracic portals, a surface area of 50.times.50 cm.sup.2 is desirable. For dental imaging, an imager as small as approximately 2.times.2 cm.sup.2 and up to approximately 3.times.4 cm.sup.2 would be clinically useful. For diagnostic radiography and fluoroscopy, imagers as large as 60.times.60 cm.sup.2 would be useful. Though solid state imagers are highly desirable, the manufacture of crystalline semiconductor detectors over such an area is certainly prohibitively expensive.
The development of a-Si:H (hydrogenated amorphous silicon) has resulted in the realization of a highly radiation resistant material which can be utilized over large surface areas at very economical cost. See V. Perez-Mendez, et al., "Signal, Recombination Effects and Noise in Amorphous Silicon Detectors", Nuclear Instrument and Methods in Physics Research A260 (1987); 195-200, Elsevier Science Publishers B.V.; and I.D. French et al., "The Effect of .gamma.-Irradiation on Amorphous Silicon Field Effect Transistors", Applied Physics A31, 19-22, 1983, Springer-Verlag.
It is now realized that amorphous silicon thin film transistors have applications to large-area electronics, see H. C. Tuan, "Amorphous Silicon Thin Film Transistor and its Application to Large-Area Electronics," Mat. Res. Soc. Symp. Proc. Vol. 33 (1984) Elsevier Science Publishing Company, Inc.
Amorphous silicon ionizing particle detectors made of hydrogenated amorphous silicon are known which can detect the presence, position and amount of high energy ionizing particles see Street et al., U.S. Pat. No. 4,785,186; however, the patent does not teach how a-Si:H photodiodes can be utilized in coordination with other elements to obtain a real-time imaging device.
Rougeot, U.S. Pat. No. 4,799,094, teaches a photosensitive device having an array of p-doped floating grids which connect with a substrate of lightly n-doped hydrogenated amorphous silicon. Since Rougeot uses transistors as light detectors, the quantity of electron-hole pairs generated would appear quite insufficient to realize real-time imaging.